1. Field of the Invention
The present invention relates to the lubrication structure of a power unit mounted on, for example, a vehicle. The power unit has a power case in which a crankcase is integrally formed with a transmission case. A crankshaft of a 4-cycle internal combustion engine is received in the crankcase. A wet type friction clutch is received in the transmission case.
2. Description of the Background Art
A conventional lubrication structure of this kind of power unit, for example, is shown in the lubrication structure disclosed in Japanese Patent Unexamined Publication No. 6-288214. In the conventional structure, a clutch case having a multiple disc friction clutch therein communicates with a transmission case having an output shaft and a transmission shaft therein which is provided with various kinds of gears. The communication is via a communication hole and an oil pan to keep the oil level in the transmission case and that of the clutch case at the nearly same level. Also, a magnet case in which a flywheel magnet is received communicates with a crankshaft case in which a crankshaft is received via the communication hole to keep the oil level in the crankshaft case and the oil level in the magnet case at the nearly same level.
The oil stored at the bottom portion of the crankshaft case is sucked, pressurized and discharged into the transmission case by a suction oil mechanism. The oil sucked from a suction port made in the transmission case is pressurized by the oil pump mechanism for lubrication, is cleaned by a oil filter, then is sent to the crankshaft, crankpin and lubricating portions in the crankcase, and then is returned to the crankcase after lubrication.
In the conventional lubrication structure, the oil stored in the transmission case, the clutch case and the oil pan is used for lubricating the lubricating portions in the crankshaft and the crankcase of the internal combustion engine, and is sucked by the suction oil pump mechanism and is returned to the transmission case. Thus, the lubrication system of the internal combustion engine and the lubrication system of the transmission shaft and the multiple disc friction clutch have a shared portion and use the same lubricating oil. Therefore, the oil lubricating the internal combustion engine is used for the lubrication system of the transmission shaft and the clutch.
However, the oil lubricating the internal combustion engine receives combustion heat and increases in temperature, and its viscosity is decreased by the increased oil temperature. Accordingly, this reduces the cooling capacity of the oil for cooling the multiple disc clutch, and thus reducing the durability of the multiple disc clutch. Therefore, in order to secure a clutch capacity, that is, a friction force necessary for good clutch connection, it is necessary to enlarge the diameter of a friction disc or to increase the number of the friction discs, which presents a problem of enlarging the clutch and thus the power unit.
Further, in the case where an additive is added to the lubricating oil in order to reduce the friction of the sliding portions of the crankshaft and a piston, the same lubricating oil is also used for the multiple disc friction clutch, and hence it produces a slip on the multiple disc friction clutch, resulting in a reduction of the connection performance thereof.
It is therefore an object of the present invention to provide a lubrication structure of a power unit which can improve the durability of the friction clutch and make the friction clutch compact and secure good connection performance of the friction clutch by separating the lubrication system of a 4-cycle internal combustion engine from the lubrication system of a wet type friction clutch.
The present invention is a lubrication structure of a power unit having a power case in which a crankcase portion for forming a crankcase, in which the crankshaft of a 4-cycle internal combustion engine is received, is integrally formed with a transmission case portion for forming a transmission case, in which a wet type friction clutch for transmitting the torque of the crankshaft is received. The crankcase and the transmission case are formed in independent cases, and the lubrication system of the internal combustion engine is separated from the lubrication system of the friction clutch.
The crankcase is made independent of the transmission case, and the flow of the lubricating oil is stopped between both cases in the integrally formed power case. In this way, the lubrication system of the internal combustion engine is separated from the lubrication system of the friction clutch. Accordingly, the lubricating oil for the internal combustion engine for lubricating the lubricating portions of the internal combustion engine, such as lubricating portions in the crankcase and others, is not mixed with the lubricating oil for the transmission mechanism supplied for the wet type friction clutch. Accordingly, since the friction clutch is not supplied with the lubricating oil for the internal combustion engine whose temperature is raised to a comparatively high temperature by the combustion heat, or which includes an additive for reducing the friction, the friction clutch is effectively cooled by the lubricating oil for the transmission mechanism, which is not heated by the combustion heat and has a comparatively low temperature, and is prevented from slipping owing to the lubricating oil for the internal combustion engine, whose viscosity is reduced by the high temperatures or which includes the additive described above, to secure a large friction force.
As a result, the following effects are made. Since the friction clutch is not supplied with the lubricating oil for the internal combustion engine whose temperature is raised to a comparatively high temperature by the combustion heat, the cooling ability of the lubricating oil is improved, and hence the durability of the friction clutch is also improved. Also, since the friction clutch is not supplied with the lubricating oil for the internal combustion engine whose temperature is raised to a high temperature to reduce its viscosity or which contains the additive for reducing the friction, it is possible to easily secure a necessary friction force and hence to realize a set clutch capacity by a compact friction clutch, and further to prevent the friction clutch from slipping and hence to obtain the friction clutch having a good connection performance. In this manner, since the friction clutch can be made compact, the power unit can also be made compact.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.